Photogelatin Printing

(also glass printing), a method of reproducing a text and simple figures in small quantities using the principles of planographic printing. A glass plate is used as the printing surface. First, a layer of gelatin is applied, and then the original, which is either typewritten or traced with special inks, is pressed to the plate. The printing images are formed as a result of the chemical reaction between the components of the gelatin and the ink of the original. Photogelatin printing is technologically a simple process, but because of its low output and the poor quality of the images, the process is giving way to printing on rotators and Multilith machines.

a method of planographic printing for the reproduction of illustrations, in which no screen is used; the process is based on the changes in the physicochemical properties of a light-sensitive layer that occur on exposure to light. Photogelatin printing was invented in 1855 by the French chemist A. Poitevin.

Plates are prepared by depositing a light-sensitive solution containing gelatin and potassium or ammonium dichromate on a foil support; the resulting layer is then dried. A halftone negative is copied onto the layer, and as a result different sections of the layer are hardened to different degrees. The plate is then washed in water to remove the unreacted salt, during which time the layer swells and acquires a reticulated structure; depressions between wrinkles constitute the printing elements of the plate. On sections of the layer that received an insignificant exposure, the wrinkles are barely noticeable and the depressions between them are shallow. During printing, ink fills only the depressions between wrinkles, and a thin layer of ink is transferred to the paper. The size of the wrinkles and the depth of the depressions between wrinkles increase with increased hardening of the emulsion; consequently, the thickness of the layer of ink to be transferred to the paper and the total printed area also increase. On the sections that undergo the greatest hardening (the dark areas of illustrations), printing ink covers the entire surface of the emulsion layer.

Reproduction of the tonal relationships in the image is accomplished by changing the thickness of the layer of ink and the dimensions of the printing elements; thus, photogelatin printing combines the features of gravure and screen processes. Piano-graphic presses with an output of up to 1,000 impressions per work shift are used for printing; the maximum run from a single plate is approximately 1,500 impressions. Rotary presses—both contact and offset—are also used. The USSR has developed a gelatin-based emulsion sensitized with dichromates that is suitable for use with regular monochrome offset presses. Aluminum sheets with a thickness of 0.6–0.8 mm or thin foil (for printing on small-format offset presses) serve as the support of the printing plate; the light-sensitive layer is deposited mechanically. The output of such presses may reach 5,000 impressions per work shift, and the maximum run is approximately 10,000 impressions.

Photogelatin printing is used for high-quality reproduction of intricate original works of art, such as pencil sketches, photographs, and oil or watercolor paintings, as well as for illustrating publications issued in small runs but requiring highly accurate reproduction of illustrations. Photogelatin printing is not widely used because of its relatively low productivity.